1. Field of the Invention
This invention relates to a method for removing mercury from a hydrocarbon fraction and more particularly, to a method for highly efficient removal of mercury from a hydrocarbon fraction which contains a small amount of mercury and can be handled in a liquid state on a commercial scale (to be referred to as "liquid hydrocarbon fraction" hereinafter).
2. Description of the Related Art
Previously, mercury removal techniques were developed and established as pollution control measures in order to remove toxic mercury from waste water and exhaust gases. A variety of techniques were available for removing mercury from water and gases.
Palladium supported on alumina and similar catalysts are often used in reforming a petroleum fraction or similar liquid hydrocarbon fraction through hydrogenation or the like. It is known that if mercury is present in the hydrocarbon fraction as an incidental impurity, the catalyst is poisoned such that modification may not fully take place.
For removal of mercury from a liquid hydrocarbon fraction, for example, Japanese Patent Application Kokai (JP-A) No. 90502/1977 discloses a method for removing mercury from vacuum pump oil by adding zinc sulfide to the oil, allowing the zinc sulfide to adsorb and collect mercury, and thereafter separating the mercury along with the excess zinc sulfide. JP-A 28295/1991 discloses use of an adsorbent in the form of active carbon having cupric chloride or stannous chloride supported thereon. JP-A 213144/1991 discloses use of an adsorbent in the form of active carbon having supported thereon halides of metal elements of Group I or II in the Periodic Table.
However, the method of JP-A 90502/1977 results in a vacuum pump oil having a mercury concentration of about 5 to 3 parts by weight per million parts by volume. This mercury removal is still insufficient. The method of JP-A 28295/1991 can be applied to only a limited range of hydrocarbon compounds. Due to the danger that copper acetylide might be formed, it is dangerous to use cupric chloride in treating a hydrocarbon which may contain acetylene. Stannous chloride to be supported on active carbon has a low solubility in water and requires the addition of hydrochloric acid in order that it may be applied or deposited to active carbon. It is difficult to remove the hydrochloric acid. As a result, the range of hydrocarbons to which this method is applicable is limited. The method of JP-A 213144/1991 has the problem that an organic mercury compound in the liquid hydrocarbon fraction is little adsorbed by the Group I or II metal halides. This method achieves a mercury removal of about 50 to 70%. It is difficult to efficiently remove mercury from the liquid hydrocarbon fraction. None of the currently available techniques can be practiced at a reasonable cost on a commercial large scale.